Electronic devices often radiate electromagnetic energy that can interfere with nearby devices. As components in an electronic device are switched on and off, the rapid changes in electrical current flow generate an electromagnetic field. This results in electromagnetic interference (EMI) to nearby devices. As electronic devices have become widespread and common, regulation of EMI has become increasingly important and has resulted in standards and regulatory limits on EMI emission. EMI management in synchronous data transmission circuits that operate on a constant frequency is particularly difficult, because the electromagnetic energy is radiated within a narrow bandwidth. Because the radiation is focused within a narrow bandwidth, it is stronger within the bandwidth and may exceed regulatory limits. For example, data storage devices such as serial advanced technology attachments (SATA) drives that transfer a stream of data with a constant clock frequency tend to focus EMI within a very narrow bandwidth.
Various techniques have been developed to manage EMI such as shielding, spread spectrum and frequency hopping. Shielding blocks EMI with grounded metal structures but is bulky and costly. Spread spectrum and frequency hopping prevent the EMI from focusing within a narrow constant bandwidth by spreading it across a wider bandwidth or continuously moving it to different frequencies. Spread spectrum is a technique that continuously modulates the frequency of a clock signal to reduce the EMI seen at any particular frequency. For example, in SATA drives the clock signal may be modulated by downspreading, lowering the clock frequency and consequently the data transmission frequency in a sawtooth fashion. Current solutions perform spread spectrum by altering the frequency of the clock source, such as by modulating the feedback divider from the voltage controlled oscillator (VCO) in a phase locked loop (PLL). However, for applications requiring independent spectrum control on multiple channels, this requires a PLL for each channel.